1. Field of the Invention
This invention relates to the electrostatic speakers, and more particularly to electrostatic speakers which include a porous stator and are capable of full audio range performance.
2. Prior Art
Audio speakers typically fall within one of two categories: dynamic or magnetic driven devices and electrostatic speakers. Dynamic speakers rely on magnetic fields operating with respect to a moving cone and magnet driven by variable electromagnetic forces corresponding to the desired audio signal. Electrostatic speakers operate within much weaker, electrostatic force fields generated from a stationary stator which carries the audio signal and drives a conductive diaphragm suspended adjacent to the stator.
Electrostatic speakers have been available for decades; however, satisfactory high fidelity reproduction has been limited to very expensive systems, typically of large surface area. These limiting factors of high cost and cumbersome size have severely limited the consumer market for electrostatic speakers as part of a general sound reproduction system. This trend is contrasted by impressive advancements in dynamic speakers, both with reduction in cost and size. As a consequence, conventional dynamic speakers comprise 99% of the total domestic market. Electrostatic speakers constitute less than 1%.
The steady decline of cost of electronic components in other fields has not been matched by electrostatic design. To the contrary, these speakers remain extremely expensive. This is due in part to the large space requirement for electrostatic speakers. Because diaphragm displacement is extremely narrow, a large diaphragm is used to achieve an adequate displacement of air to develop desired amplitude, particularly at lower frequencies. In view of the required large diaphragm area, design and construction of drive systems and enclosures has tended to develop complexities in providing a uniform stator and corresponding diaphragm continuity.
One common element of electrostatic speakers is a rigid stator. The stator must be conductive to provide the variable voltage with attendant audio signal for driving the diaphragm. The rigidity of the stator is significant because the diaphragm must be maintained in a taut configuration to be fully responsive to the variations in electrostatic field strength carrying the audio signal. Any occurrence of nonuniformity in tension in the diaphragm may lead to nonlinear response in speaker output. Accordingly, the conventional stator typically bears the stress of tension applied around the total perimeter of the diaphragm.
Prior art stator elements have included rigid screens and grids, as well as perforated conductive plates. See, for example, U.S. Pat. No. 3,008,013 of Williamson et al, and U.S. Pat. No. 3,892,927 of Lindenberg. Electrical contacts are provided on the stator for coupling leads from the voltage source. Perforations or open screen and grid structure enable acoustic transparency, meaning passage of sound waves through the stator from the diaphragm to the surrounding environment. Variations in openings sizes and shapes in stator plates is clearly shown in the various patent cited above. Such plates include molded or stamped perforations which range in dimensions up to several centimeters.
The diaphragm is placed in tension across the interior surfaces of the stators to provide a surface which can vibrate as a speaker element. Numerous complex configurations are illustrated for tensing or stretching the diaphragm across the stator to realize appropriate resonant frequencies needed for predictable sound reproduction. Prior art devices have shared the common feature of full perimeter tension wherein all parts of the periphery of the diaphragm have some level of attachment with the stators which includes an element of tension or stress applied along a transverse diagonal connecting opposing diametric edges of the diaphragm.
The amount of applied tension is a function of desired resonant frequency. Because the diaphragm operates in a manner similar to a taut drum head, there exists at least one resonant frequency for the diaphragm which will tend to distort an otherwise linear response to the diaphragm as an audio speaker element. In addition to this characteristic resonant frequency, the diaphragm experiences a bass roll off effect which is associated with the diameter of the diaphragm. This is represented in FIG. 1 and occurs at a knee 10 of the response curve, dropping off at 6 db per octave. These properties of speaker diaphragms impose difficult design problems with respect to high fidelity sound reproduction within an electrostatic system.
The various patent previously cited offer different techniques for compensating for one or both of these frequency variations. For example, a U.S. patent issued to Winey isolates sections of the diaphragm with different physical dimensions to create variations in the resonant frequency for the speaker, including a high frequency strip along a length of the speaker. In a different U.S. patent issued to West, the patent teaches a similar strategy of placing damping barriers to divide the diaphragm into sections with different resonant frequencies which are selected to offset and reinforce losses associated with the referenced roll off phenomenon.
Those skilled in the art will be familiar with other limitations within electrostatic speakers which have inhibited commercialization of systems which are cost competitive with conventional dynamic speakers. The previous discussion is simply for the purpose of demonstrating representative technical difficulties which have challenged the electrostatic speaker industry. What is clear is that electrostatic speakers have been unable to keep pace with the continued expansive growth of dynamic speaker systems.